Some observations on habituation of the flexor reflex in the rat: the influence of strychnine, bicuculline, spinal transection, and decerebration

The involvement of inhibition in habituation of the flexor reflex was investigated in intact, spinal, and decerebrate rats. Strychnine and bicuculline were administered in order to determine the contribution of certain forms of central inhibition to the development of habituation. Both strychnine and bicuculline reduced habituation in the intact rat but did not do so in the spinal preparation. Strychnine, in fact, caused a facilitation of habituation in the spinal rat. The impairment of habituation by strychnine was related to the intensity of stimulation used to elicit the reflex. Thus, the effect of strychnine was only demonstrable when relatively high intensities were used. Flexor reflex habituation was shown to be more pronounced in the decerebrate than in the spinal preparation. This difference could be demonstrated only when stimuli of high intensity were given. It is concluded that, in the absence of supraspinal influences, habituation of the flexor reflex does not require inhibitory mechanisms. However, inhibition may play a role in habituation of the component of the reflex that utilizes descending influences.     

The cerebellum and initiation of movement: the stretch reflex

Studies of the stretch reflex in decerebrate cats indicate a phase advance of peak sinusoidal tension in steady-state cycles between 0.1 and 10 Hz. This phase advance is reduced in acute and chronic cerebellectomy, as shown in previous investigations. Also, the augmentation of muscle peak tension in initial sinusoidal stretch cycles at 0.5-5 Hz has been found to be reduced during the time of reflex and motor instability in the several months following cerebellar ablation. This report shows the increased amplitude and phase lead of integrated electromyographic activity in initiating sinusoidal stretch cycles in the decerebrate cat. These reflex aspects are demonstrated in relation to the discharge of neurons in the dorsal spinocerebellar tract and of cerebellar cortical Purkinje cells in initial sinusoidal cycles. The intensity and phase advance of the discharge in dorsal spinocerebellar tract neurons is altered little, but these features are usually increased in Purkinje cells during initial stretches compared to continuous cycling. In terms of overall motor control, these findings are compatible with concepts of movement control, modulated by the cerebellum, in which the discharge of antagonist motor neurons is regulated in concert with that of agonist muscles upon initiation and termination of movement.     

High concentrations of 17beta -estradiol attenuate the exercise pressor reflex in male cats.

Previously, intravenous injection of 17beta-estradiol in decerebrate male cats was found to attenuate central command but not the exercise pressor reflex. This latter finding was surprising because the dorsal horn, the spinal site receiving synaptic input from thin-fiber muscle afferents, is known to contain estrogen receptors. We were prompted, therefore, to reexamine this issue. Instead of injecting 17beta-estradiol intravenously, we applied it topically to the L(7) and S(1) spinal cord of male decerebrate cats. We found that topical application (150-200 micro l) of 17beta-estradiol in concentrations of 0.01, 0.1, and 1 micro g/ml had no effect on the exercise pressor reflex, whereas a concentration of 10 micro g/ml attenuated the reflex. We conclude that, in male cats, estrogen can only attenuate the exercise pressor reflex in concentrations that exceed the physiological level.     

Estrogen attenuates the exercise pressor reflex in female cats.

In humans, the pressor and muscle sympathetic nerve responses to static exercise are less in women than in men. The difference has been attributed to the effect of estrogen on the exercise pressor reflex. Estrogen receptors are abundant in areas of the dorsal horn receiving input from group III and IV muscle afferents, which comprise the sensory limb of the exercise pressor reflex arc. These findings prompted us to investigate the effect of estrogen on the spinal pathway of the exercise pressor reflex arc. Previously, we found that the threshold concentration of 17beta-estradiol needed to attenuate the exercise pressor reflex in male decerebrate cats was 10 microg/ml (Schmitt PM and Kaufman MP. J Appl Physiol 94: 1431-1436, 2003). The threshold concentration for female cats, however, is not known. Consequently, we applied 17beta-estradiol to a well covering the L6-S1 spinal cord in decerebrate female cats. The exercise pressor reflex was evoked by electrical stimulation of the L7 or S1 ventral root, a maneuver that caused the hindlimb muscles to contract statically. We found that the pressor response to contraction averaged 38 +/- 7 mmHg before the application of 17beta-estradiol (0.01 microg/ml) to the spinal cord, whereas it averaged only 23 +/- 4 mmHg 30 min after application (P < 0.05). Recovery of the pressor response to contraction was not obtained for 2 h after application of 17beta-estradiol. Application of 17beta-estradiol in a dose of 0.001 microg/ml had no effect on the exercise pressor reflex (n = 5). We conclude that the concentration of 17beta-estradiol required to attenuate the exercise pressor reflex is 1,000 times more dilute in female cats than that needed to attenuate this reflex in male cats.     

The effects of cold-induced muscle spindle secondary activity on monosynaptic and stretch reflexes in the decerebrate cat.

The effects of muscle spindle secondary ending activity on the stretch reflex were studied in unanesthetized decerebrate cats. Activation of secondary endings was accomplished by reducing the muscle temperature. This has been shown to cause a sustained asynchronous discharge from secondary endings. Cooling of the medial gastrocnemius or lateral gastrocnemius-soleus muscles caused an increase in the phasic and tonic components of their stretch reflexes. Cooling of the relaxed medial gastrocnemius muscle caused similar increases in the components of the stretch reflex of the synergistic lateral gastrocnemius-soleus muscle and an increase in its monosynaptic reflex. It was concluded that the facilitatory autogenetic and synergistic effects of muscle cooling on the stretch and monosynaptic reflexes were brought about by activity in group II afferents from muscle spindle secondary endings and could not be ascribed to any other type of muscle receptor. These results support the concept of an excitatory role for the secondary endings of the muscle spindle in the stretch reflex of the decerebrate cat.     

Intrathecal cholecystokinin interacts with morphine but not substance P in modulating the nociceptive flexion reflex in the rat

The effect of intrathecal (IT) cholecystokinin (CCK), substance P (SP) and morphine (MO) on spinal cord excitability was studied in decerebrate, spinalized rats. CCK had a weaker facilitatory effect on the nociceptive flexion reflex than SP. The possible functional significance of the coexistence of CCK and SP in neurons projecting to the spinal cord was tested by coadministration of the two peptides. At the doses tested no synergistic interaction on the reflex was found with CCK and SP. IT MO caused a brief enhancement followed by a prolonged depression of the reflex. A high dose of CCK injected prior to MO increased the facilitatory effect and decreased the depressive effect of the opiate on the reflex. The effect of desulfated (D) CCK was similar to CCK but at a higher dose. Naloxone (NAL) had a similar effect as CCK when administered prior to MO. The MO-induced depression of the reflex was readily reversed by NAL, but not by CCK. The results indicate that CCK may prevent the inhibitory effect of MO on spinal cord excitability to nociceptive stimulation, but does not reverse it. CCK may alter the balance of excitation-inhibition between various types of dorsal horn interneurons that are involved in the transmission of nociceptive information.     

Modulation of dynamic parameters of muscle reflex by selective activation of its gamma system

The ability of the gamma system to modify the dynamics of the muscle reflex control system is investigated. The dynamics of the triceps surae muscle in decerebrate cat preparations with intact reflex loops were modulated by contralateral tibial and peroneal nerve stimulation. The parameters of the mathematical models representing the muscle reflex system were estimated by the least squares method. The behaviour of the mathematical models of the system, under various degrees of gamma system stimulation, was then studied in response to disturbance inputs. It is shown that the gamma motor system is an efficient agent for carrying out modifications in the dynamics of the muscle reflex system. The possible functional significance of this phenomenon within the framework of operation of the muscular control system is discussed with reference to optimal adaptive system concepts.     

Effect of phenamine and gutimine and gutimine on adaptive changes in arterial pressure occurring during skeletal muscle contraction]

Effects of amphetamine (0,1--11 mg/kg) and gutimin (5--50 mg/kg) on the pressor reflex originating in the exercising muscle were studied on nonanesthetized decerebrate cats. Isometric exercise of the hind limb muscles was elicited by stimulating the spinal ventral roots L6--S1. Amphetamine (0.1 mg/kg) increased the level of arterial pressure and the amplitude of the pressor reflex. Higher doses of amphetamine decreased the pressor reflex to exercises and there was a fall of arterial pressure. Gutimin elevated the arterial pressure and failed to alter the pressor reflex to exercises.     

The spinocervical tract as a possible pathway for muscular nociception.

In decerebrate and chloralose-anaesthetized spinal cats, micro-electrode recordings were made from axons of the spinocervical tract (SCT) during intra-arterial injection of bradykinin, 5-hydroxytryptamine and potassium into the gastrocnemius-soleus (GS) muscle. The results suggest that muscular group IV and group III afferent units which respond to the above painful stimulation project into the SCT. The portion of SCT units that could be activated by injections of these algesic substances into the GS-muscle was different in spinal and decerebrate preparations, showing that the SCT neurones are subject to a descending influence. It is concluded that the spinocervical tract, in addition to its function as a cutaneous sensory pathway, may serve as a pathway for muscular nociception in the cat.     

Role of spinal NMDA and non-NMDA receptors in the pressor reflex response to abdominal ischemia

Abdominal ischemia induces a pressor reflex caused mainly by C-fiber afferent stimulation. Because excitatory amino acids, such as glutamate, bind to N-methyl-D-aspartate (NMDA) and non-NMDA [dl-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)] receptors and serve as important spinal neurotransmitters, we hypothesized that both receptors play a role in the abdominal ischemia pressor reflex. In chloralose-anesthetized cats, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate did not alter the pressor reflex (33 +/- 9 to 33 +/- 7 mmHg, P > 0.05, n = 4), whereas AMPA receptor blockade with 4.0 mM 6-nitro-7-sulfamylbenzo(f)quinoxaline-2,3-dione significantly attenuated the reflex (29 +/- 5 to 16 +/- 4 mmHg, P < 0.05, n = 6). Because several studies suggest that anesthesia masks the effects of glutamatergic receptors, this experiment was repeated on decerebrate cats, and in this group, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate significantly altered the pressor reflex (36 +/- 3 to 25 +/- 4 mmHg, P < 0.05, n = 5). Our combined data suggest that spinal NMDA and AMPA receptors play a role in the abdominal ischemia pressor reflex.     

The effect of intraspinal microinjection of dopamine on the C-fiber reflex in the acute decerebrate spinal cat.

Dopamine microinjection (10 μg) into the ventral spinal cord gray matter of the L₇ segment of the spinal cord facilitated the C-fiber reflex; however, facilitation occurred after a latency of 15 min. In contrast norepinephrine (10 μg) microinjections facilitated the C-fiber reflex after a shorter latency (2 min). Treatment of the cat with a dopamine ß-hydroxylase inhibitor prevented the facilitatory effect of dopamine. These observations indicate that microinjected dopamine facilitates C-fiber reflexes by serving as a precursor for norepinephrine.     

Changes in the excitability of and synaptic effects on motoneurons during formation of the scratch motion generator in the cat

Studies on immobilized decerebrate (at intracollicular level) cats in which the scratch generator had been set up following bicuculline application to the upper cervical segments of the spinal cord, showed that the state of the segmental apparatus of the lumbosacral section of the spinal cord differs substantially from that seen in the spinal animal. Direct excitability of motoneurons of the "aiming" and "scratching" muscles rises, while recurrent and reciprocal Ia inhibition of motoneurons intensifies and the influence of Ib afferents on motoneurons declines. Afferents of the flexor reflex exert a primarily inhibitory influence on motoneurons of the "aiming" muscles. This influence becomes predominantly excitatory following spinalization, while the inhibitory effects of these afferents on motoneurons of the "scratch" muscles declines. The functional significance of the changes discovered in generation of scratch routine is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 2, pp. 244–250, March–April, 1987.     

The response of alpha-motoneurons of different size to stretch and vibration of extensor muscles after injection of 5-hydroxytryptophan in spinal cats.

1. Vibration of the GS muscle at 100/sec, and the peak-to-peak amplitude of about 100 mu, produced slight reflex activity of homonymous and heteronymous alpha-motoneurons in the spinal, unanesthetized cat. 2. Intravenous injection of the 5-HT precursor 5-HTP (25 mg/kg, L-form) performed in the spinal preparation facilitated the responses of single extensor alpha-motoneurons to both vibration and stretch of the corresponding muscle. The 5-HT antagonist methysergide (0-5 mg/kg i. v.) blocked almost completely the facilitation of the vibration and stretch induced motoneuronal discharges produced by 5-HTP in the spinal cat, indicating that the facilitation was 5-HT specific. 4. A comparison of the types of alpha-motoneurons participating in the reflex responses elicited by the different stimuli in the spinal cat following injection of 5-HTP showed that both small-tonic and large-phasic alpha-motoneurons take part in both stretch and vibration reflexes. 5. It is concluded that the alpha-motoneuron pool of the GS muscle represents an homogenous population, no matter what kind of synaptic drive is used for its activation, and that the susceptibility of a motoneuron to discharge depends on the amount of the afferent excitatory input rather than on the modalities of sensory stimulation.     

Physiologic evidence for descending tryptaminergic pathways in the spinal cord.

LSD was more effective in facilitating the flexor reflex in the chronic than in the acute spinal dog. On the other hand, $\text{1}$-tryptophan produced a significant facilitation of the flexor reflex in the acute spinal dog but not in the chronic spinal dog. These data are consistent with the hypothesis that there are long descending tryptaminergic axons which have the capacity of converting tryptophan to tryptamine and releasing it in the vicinity of post-synaptic facilitatory receptors which degenerate in the chronic spinal dog.     

Exercise pressor reflex function in female rats fluctuates with the estrous cycle

In women, sympathoexcitation during static handgrip exercise is reduced during the follicular phase of the ovarian cycle compared with the menstrual phase. Previous animal studies have demonstrated that estrogen modulates the exercise pressor reflex, a sympathoexcitatory mechanism originating in contracting skeletal muscle. The present study was conducted in female rats to determine whether skeletal muscle contraction-evoked reflex sympathoexcitation fluctuates with the estrous cycle. The estrous cycle was judged by vaginal smear. Plasma concentrations of estrogen were significantly (P < 0.05) higher in rats during the proestrus phase of the estrus cycle than those during the diestrus phase. In decerebrate rats, either electrically induced 30-s continuous static contraction of the hindlimb muscle or 30-s passive stretch of Achilles tendon (a maneuver that selectively stimulates mechanically sensitive muscle afferents) evoked less renal sympathoexcitatory and pressor responses in the proestrus animals than in the diestrus animals. Renal sympathoexcitatory response to 1-min intermittent (1- to 4-s stimulation to relaxation) bouts of static contraction was also significantly less in the proestrus rats than that in the diestrus rats. In ovariectomized female rats, 17β-estradiol applied into a well covering the dorsal surface of the lumbar spinal cord significantly reduced skeletal muscle contraction-evoked responses. These observations demonstrate that the exercise pressor reflex function and its mechanical component fluctuate with the estrous cycle in rats. Estrogen may cause these fluctuations through its attenuating effects on the spinal component of the reflex arc.     

Inhibition of spinal reflex responses in the last period of embryogenesis

Some characteristics of spinal reflex reaction inhibition were studied in cat fetuses during the last three weeks of antenatal development. The experiments were conducted on fetuses with intact placental circulation. Restoration of the excitability of the spinal reflex arcs was very slow after stimulation of the dorsal root by a single stimulus. In embryos studied 20 days before birth the full inhibition of reflex responses lasted about 500 msec. Even 2–3 sec after a single stimulation of the afferent fibers the amplitude of the reflex response to the second stimulus was only 30–40% of the control value. It was determined that such long postactivation depression is unrelated to refractoriness or antidromic inhibition. The presence of a prolonged intense depolarization of afferent terminal fibers at these stages suggests a presynaptic inhibition as one of the most probable reasons for the prolonged postactivation depression. Another important factor in the appearance of postactivation depression is probably the morphologic and functional immaturity of synaptic structures. A reciprocal inhibition was observed in cat fetuses on the 10–12th antenatal day. On the basis of these data it is suggested that in embryogenesis presynaptic inhibition considerably precedes that of postsynaptic fibers.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 68–75, January–February, 1971.     

Stochastic versus deterministic variability in simple neuronal circuits: I. Monosynaptic spinal cord reflexes.

Long time series of monosynaptic Ia-afferent to alpha-motoneuron reflexes were recorded in the L7 or S1 ventral roots in the cat. Time series were collected before and after spinalization at T13 during constant amplitude stimulations of group Ia muscle afferents in the triceps surae muscle nerves. Using autocorrelation to analyze the linear correlation in the time series demonstrated oscillations in the decerebrate state (4/4) that were eliminated after spinalization (5/5). Three tests for determinism were applied to these series: 1) local flow, 2) local dispersion, and 3) nonlinear prediction. These algorithms were validated with time series generated from known deterministic equations. For each experimental and theoretical time series used, matched time-series of stochastic surrogate data were generated to serve as mathematical and statistical controls. Two of the time series collected in the decerebrate state (2/4) demonstrated evidence for deterministic structure. This structure could not be accounted for by the autocorrelation in the data, and was abolished following spinalization. None of the time series collected in the spinalized state (0/5) demonstrated evidence of determinism. Although monosynaptic reflex variability is generally stochastic in the spinalized state, this simple driven system may display deterministic behavior in the decerebrate state.     

Post-tetanic potentiation of polysynaptic reflexes of the spinal cord

The phenomenon of post-tetanic potentiation has been studied in the cat spinal cord with particular reference to polysynaptic responses. Following tetanization of dorsal roots, these reflexes show an increased response, as measured in terms of their voltage-time area, with a predominant change in the earlier reflex pathways. Both of these changes in the reflex discharge have a time course of 15 to 25 seconds. Post-tetanic potentiation is also observed in response to stimulation of a dorsal rootlet following tetanization of another rootlet in the same or in a neighboring segment. This effect can be explained by post-tetanic changes in the terminals of secondary, and possibly higher order, internuncial cells, essentially similar to those changes in the primary afferent terminals which give rise to potentiation of the monosynaptic reflex.     

Late long-lasting discharges in hind limb motor nerves and their connection with the locomotor rhythm in thalamic immobilized cats

Effects of stimulation of flexor reflex afferents were studied in decerebrate immobilized cats. Stimulation of ipsilateral afferents evoked late long-lasting discharges in the nerves to the flexors, whereas stimulation of contralateral afferents led to similar discharges in nerves to both extensors and flexors. Compared with spinal animals, early segmental reflexes in thalamic cats were tonically depressed. Similar tonic inhibition of segmental reflexes took place in spinal animals after injection of dopa. Segmental reflexes were clearly modulated during late or rhythmic discharges. The possible central mechanisms of these changes in the segmental reflexes are discussed on the basis of data in the literature.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 2, pp. 137–145, March–April, 1979.     

Mechanisms of reflexes induced by esophageal distension

We investigated the mechanisms of esophageal distension-induced reflexes in decerebrate cats. Slow air esophageal distension activated esophago-upper esophageal sphincter (UES) contractile reflex (EUCR) and secondary peristalsis (2P). Rapid air distension activated esophago-UES relaxation reflex (EURR), esophago-glottal closure reflex (EGCR), esophago-hyoid distraction reflex (EHDR), and esophago-esophagus contraction reflex (EECR). Longitudinal esophageal stretch did not activate these reflexes. Magnitude and timing of EUCR were related to 2P but not injected air volume. Cervical esophagus transection did not affect the threshold of any reflex. Bolus diversion prevented swallow-related esophageal peristalsis. Lidocaine or capsaicin esophageal perfusion, esophageal mucosal layer removal, or intravenous baclofen blocked or inhibited EURR, EGCR, EHDR, and EECR but not EUCR or 2P. Thoracic vagotomy blocked all reflexes. These six reflexes can be activated by esophageal distension, and they occur in two sets depending on inflation rate rather than volume. EUCR was independent of 2P, but 2P activated EUCR; therefore, EUCR may help prevent reflux during peristalsis. All esophageal peristalsis may be secondary to esophageal stimulation in the cat. EURR, EHDR, EGCR, and EECR may contribute to belching and are probably mediated by capsaicin-sensitive, rapidly adapting mucosal mechanoreceptors. GABA-B receptors also inhibit these reflexes. EUCR and 2P are probably mediated by slowly adapting muscular mechanoreceptors. All six reflexes are mediated by vagal afferent fibers.